Discussion on the most reasonable selection of dis

2022-09-19
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Discussion on reasonable selection of distribution equipment capacity

1 The selection of distribution transformer

according to the international jgj/t16-92): "those with electric equipment capacity of 250KW or requiring transformer capacity of more than 160KVA should be powered by high voltage." Requirements. It can be seen that all projects with a certain building scale will use power transformers. However, there are still misunderstandings for some designers on how to choose the transformer capacity. It is considered that the active load capacity and capacity of transformer should be selected according to the calculated load or close to full load. In fact, this is an illusion, mistakenly believing that "full load" can make the best use of everything and save investment. However, although the transformer is an electrical equipment with an efficiency of more than 95% (n=p output/input x100:95%). But only when the load of the transformer is 0 6, which is also the primary condition and basis for selecting transformer capacity from the perspective of giving full play to the highest efficiency of the transformer

of course, other factors should be comprehensively considered when finally determining the transformer capacity. For example, the influence of ambient temperature, reducing the temperature can improve the output power of the transformer and reduce the loss of the transformer, and the reasonable selection of the number of transformers and technical and economic comparison are all factors that affect the selection of transformer capacity

as for the overload capacity of the transformer, it is related to the factors related to the initial load rate, ambient temperature, ventilation and heat dissipation conditions, and can only be emergency and short-term 5 Between real-time data collection. In case of overload, it is first required not to damage the insulation of the transformer and reduce the use efficiency. The peak power consumption in the middle of the year may be overloaded, and the light load operation will occur at the bottom. The quantity and time between "overload" and "light" load will basically play a complementary role at the same time, but it is best not to overload%

calculation formula of overload percentage (n): n= (i-le)/iex100%, where: I - actual load current of transformer; Ie- rated current of transformer

of course, for the transformer equipped with forced air cooling, its emergency overload capacity can reach 40% - 50%, and the interruption time can be appropriately extended (but long-term operation under overload is not allowed), which can be determined by the technical conditions of the product

considering the influence of the above factors on the selection of transformer capacity, from the perspective of energy conservation, economy, practicality, safety and reliability, it is generally appropriate to select the transformer load rate between 0.65 and 0.8

2. Selection and device of capacitor reactive power compensation capacity

at present, China requires the power factor of power users to be more than 0.9 for high-voltage power supply and more than 0.85 for low-voltage power supply. For this reason, most industrial and civil buildings adopt the method of compensation, that is, install several groups of power capacitors on the distribution bus of the low-voltage distribution room to compensate the reactive power within the power supply range, so as to achieve the purpose of improving the power factor. However, some designers make mistakes on how to correctly select the capacitor capacity 6 Monitoring the experimental process: the experimental force, deformation, displacement, curve and other parameters in the experimental process can be displayed in real time. It is considered that when making the construction drawing design, it is only necessary to select the compensation capacity (kvar) according to 1/3 of the distribution transformer capacity (kVA)

for example, when the transformer capacity is 1600kav, it is obviously incorrect to select the capacitor compensation capacity as 1600/3=533 (kvar), and provide customers with efficient local product supply at the same time. It can be seen from the vector of the power triangle that in order to increase the power factor from cost to Cos2, the capacity of the compensation capacitor must be installed:

qc-ql-q2=p (tg1-tg2)

where: QC - set the compensation capacity of the capacitor, kvar; P - total active calculated load, kW; TGL and TG2 correspond to the tangent values of COSL and Cos2 before and after compensation, and the corresponding values of COS and TG can also be found out through the trigonometric function table

from the above calculation and analysis, it can be seen that the reactive compensation capacity of the capacitor is related to the load nature and load (active and reactive) size of the electrical equipment, and is calculated, but has nothing to do with the capacity of the transformer

in recent years, a reactive power compensation dry-type capacitor with self-healing function and built-in discharge resistance, small size, low dielectric loss, fire prevention, reliability and high safety performance has replaced the old oil immersed capacitor. For example, domestic models mainly include bmmj models, and imported products include ABB's CLMD models

clmd capacitor has the characteristics of high capacity and fast discharge speed. The single capacity can reach 83kvar, and the discharge speed can drop to 50V one minute after the capacitor leaves the power supply

reactive power compensation capacitor will produce dozens or even hundreds of times of rated current inrush on the distribution line at the moment of putting into operation, so it is required to install a special capacitor removal contactor on the distribution line

the characteristic of this contactor is that it is equipped with a resistance inside the product to limit the inrush current and force the capacitor to discharge, so that it can limit the inrush current within 20 times the rated power supply of the capacitor and ensure the maximum residual voltage of its terminal when the capacitor is put into operation next time

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